Laser imaging systems have found wide applicability in the reproduction of digital data and digital images on hard copy, such as paper or film. Typically, either a gas laser or a diode laser is used as the laser source.
Modulation of the gas laser may be effected by means of an acousto-optic modulator which modulates the laser beam produced by the gas laser. When a laser diode is the laser source, it may be modulated directly by the input digital signal. However, in both cases, the dynamic range of the laser imaging system is limited by either the acousto-optic modulator or the diode laser. A typical acousto-optic modulator has a dynamic range of 300:1, while a typical diode laser has a dynamic range of 150:1. These dynamic ranges may be sufficient for normal laser printer/copier applications such as disclosed in U.S. Pat. No. 4,379,631, issued Apr. 12, 1983, entitled "Apparatus Having a Copier Function and a Printer Function", and U.S. Pat. No. 4,783,680, issued Nov. 8, 1988, entitled "Halftone Screening System for Printer/Copier". Both of these patents disclose electrophotographic systems using either a directly modulated laser diode or a gas laser whose output beam is modulated by means of an acousto-optic modulator.
These limited dynamic ranges are not large enough to cover the whole dynamic range of the recording film (which is typically about 1,000:1) when high contrast radiographic images are printed. For example, in the printing of a radiographic image, the maximum density of the original image can be as high as 4.0. Due to the limited dynamic range of the acousto-optic modulator, a currently available radiographic laser printer can only reproduce images with 2.8 maximum density. As a result, some valuable diagnostic information is lost and image quality is degraded. There is thus a need in the field of laser printers for a laser printer which utilizes the full dynamic range of the print film and for a technique for extending the dynamic range of the laser printer.